Final answer:
The iris is the muscular membrane that regulates the amount of light entering the eye by constricting or dilating the pupil through the pupillary light response.
Step-by-step explanation:
The muscular membrane that regulates the dilation of the eye to control the amount of light entering is called the iris. The iris is an iris-pigmented, circular muscle located at the front of the eye. Under bright light conditions, the pupillary light response is activated, where the pupils constrict to limit light exposure to the retina. This is an autonomic reflex that involves sensory input through the optic nerve and motor response through the oculomotor nerve to the circular fibers of the iris. When light levels are low, the sympathetic nervous system takes over, dilating the pupils to allow more light in.